The present invention relates to a video tape recorder, and more particularly to a circuit for automatically controlling the carrier frequency of the video tape recorder.
The video tape recorder generally converts the luminance signals into FM (frequency modulation) signals which are recorded on a tape. The video tape recorder employs an external variable resistor to correctly modulate the luminance signals into a carrier frequency of 3.4 MHZ for a sync tip level of the luminance signals. The external variable resistor is adjusted to control the carrier frequency to compensate for the characteristic changes of the frequency modulator.
On the other hand, a conventional PLL (Phase Locked Loop) compares a free oscillation frequency of a voltage controlled oscillator (VCO) with a frequency of a reference signal applied to a phase detector to produce an error signal that is changed into an error voltage through an integrating circuit such as a low pass filter (LPF). The error voltage is fed back to the VCO to generate a same frequency as a frequency of the reference signal.
The conventional PLL is limited in causing the voltage controlled oscillator to only generate the same frequency as that of the reference signal applied to the phase detector, and therefore only a single frequency can be controlled. Moreover it is difficult to manually adjust the carrier frequency without error, and the external variable resistor results in voltage loss.